1. Field of the Invention
The present invention relates to an ultrasonic vibration driven motor including a bar-shaped vibration member and a movable body press-contacted to the vibration member. Electric energy is supplied to an electric-mechanical energy converting element provided on the bar-shaped vibration member, such that surface portions of the vibration member achieve circular or elliptical motions. In this manner, the movable body press-contacted to the vibration member is friction-driven, thus generating motor output. Such an ultrasonic vibration driven motor achieves quiet operation and is highly responsive. Therefore, it has particular utility when used to operate a camera lens system, and further may be used in a wider variety of fields and apparatuses, such as information processing apparatuses.
2. Description of the Related Art
FIG. 2 illustrates a stack arrangement of piezoelectric elements and an electric polarization pattern of the piezoelectric elements used in a bar-shaped ultrasonic vibration driven motor (hereinafter, referred to as "bar-shaped vibration driven motor"). As indicated in FIG. 2, each piezoelectric element is divided into halves by a portion located at a center line, and the two halves are oppositely polarized.
The five piezoelectric elements consist of two A-phase elements, two B-phase elements and one S-phase element. The A-phase and B-phase piezoelectric elements are arranged so as to have a 90.degree. phase difference. The S-phase piezoelectric element is disposed at the bottom of the stack and is used to detect a resonance vibration. Although not shown in FIG. 2, when assembled, electrode plates are inserted between the piezoelectric elements.
Operation of the bar-shaped vibration driven motor will be described with reference to FIG. 3.
When only the A-phase piezoelectric elements are supplied with AC voltage, they repeatedly contract and expand and, thus, a vibration member 1 comprising vibration member components 1c, 1d achieves a primary bending principal vibration in a right-left direction (as shown in FIG. 3). Similarly, when only the B-phase piezoelectric elements are supplied with AC voltage, the vibration member 1 vibrates in a plane perpendicular to the plane of the sheet. If the vibration by the A-phase and the vibration by the B-phase are provided with a 90.degree. phase difference, then the vibration member 1 achieves clockwise or counterclockwise circular motion about the lengthwise axis of the vibration member.
The vibration member 1 has a circumferential groove 1a for enhancing displacement caused by the vibration, such that the end portion of the vibration member oscillates in a circular motion, as indicated in FIG. 3. When viewed from above the contact surface (the top surface of the vibration member 1), this oscillating vibration is regarded as a single-wave progressive wave. If a rotor 2 having a contact spring portion is press-contacted to the vibration member 1 at the top end thereof, then the rotor contacts a portion on the top end corresponding to the crest of the progressive wave, and is thereby driven to rotate in a direction opposite the direction of the circular oscillation of the vibration member 1. A drive output is extracted by a gear 4 provided around a ball bearing 3 which is provided at an upper portion of the rotor 2.
In general, a bar-shaped vibration driven motor is designed on the basis of the FEM analysis of the characteristic mode of a combined assembly of the vibration member 1, a supporting pin shaft 5 (the shaft end) and a flange portion 6, so as to reduce the vibration amplitude of the flange portion 6. Therefore, bar-shaped vibration driven motors experience considerably less supporting loss, compared with ring-shaped vibration driven motors.
A rotor contact spring 7 is formed in a lower portion of a rotor main ring 2a of the rotor 2 of the bar-shaped motor driven motor. The shape of the rotor contact spring 7 provides elasticity. Also, like the rotor contact spring of a ring-shaped vibration driven motor, the rotor contact spring 7 of the bar-shaped vibration motor is designed to have a natural frequency substantially higher than the excitation frequency of the vibration member 1, and thereby follows the vibration. Further, the rotor main ring 2a has a large inertial mass and, therefore, remains unexcited even when the vibration member 1 is excited.
Advantages of a vibration driven motor include a small-size body and a large torque output. Also, a vibration driven motor normally requires no speed-reducing gear or, if such a gear is required, then it generally requires only a small speed reduction ratio. Therefore, such a motor has particular utility in a small-size apparatus that must achieve quiet operation. However, in such applications, further size reduction and torque enhancement in vibration driven motors are desired.
To achieve a large torque output, it is desirable to provide the rotor and the vibration member with large vibration diameters.
Further, to output a large torque, the contact pressure may be increased. However, because the component parts of the rotor supporting system and the like are inevitably made smaller and thinner as a result of the size reduction of the motor, a large contact pressure between the rotor and the vibration member is likely to deform such component parts. The deformation of such component parts will result in degradation of the performance of the motor, for example, fluctuation of the contact pressure therebetween. In addition, an increase in the contact pressure may well reduce the service life of the bearing.
Therefore, to achieve both size reduction and torque enhancement, it is preferable that the contact portion of the rotor be adjacent to the outermost periphery of the rotor.
To reduce sliding loss, the rotor contact spring 7 must be designed to substantially prevent undesired slippage, that is, any slippage unnecessary for driving.
Part of such undesired slippage is radial slippage. As shown in FIG. 4, the vibration member 1 exhibits, at a rotor contacting portion, a displacement .DELTA.z in the axial direction, and a displacement .DELTA.r in the radial direction.
In a conventional vibration driven motor, a contact spring 7 provided on the rotor 2 is formed in the shape of a letter "L", as shown in FIG. 5. The "L"-shaped contact spring 7 pivotably bends substantially about a point A.sub.0, so as to achieve displacements .DELTA.z and .DELTA.r of the contact portion P.sub.0, thus preventing radial slippage.
However, because the contact portion is positioned at a location radially inward from the outermost periphery of the rotor 2, this construction has a drawback in that reduction of the diameter of the rotor 2 may be limited.
To overcome this drawback, a rotor contact spring 7 may extend outward from the rotor main ring 2a, as shown in FIG. 6. However, because the direction of the pivotable bending of the contact spring 7 about a point A.sub.1, more specifically, the direction of the displacement of the contact point P.sub.1, differs greatly from the direction of the displacement of the vibration member, it is difficult in this construction to provide a contact spring which prevents radial slippage and has a suitable spring hardness with respect to the axial direction.
A construction as shown in FIG. 7 has been proposed, in which a vibration member has a contact spring 1b. This construction facilitates achieving the coincidence of the displacing direction of the contact portion including a peripheral point P.sub.2, which pivotably moves about a point A.sub.2, with the displacing direction of the vibration member, thereby eliminating radial slippage.
However, this construction has drawbacks as follows.
The contact spring 1b smoothly contacts the vibration member if a peripheral point, for example, the point P.sub.2, on the contact spring 1b is displaced as indicated by the graph in FIG. 8, where fr is the driving frequency. To achieve such displacement, the contact spring 1b must be responsive to a frequency at least twice the motor driving frequency and, preferably, an even higher frequency.
For the sake of size reduction, the vibration member preferably has a reduced axial length. However, a reduction in the axial length increases the driving frequency. Therefore, in order to maintain a low-speed rotation of the vibration driven motor, which is one of the features thereof, despite the reduced axial length, the amplitude must be restricted to a small range, resulting in strict tolerance requirements in machining conditions, such as surface precision.
To reduce the characteristic frequency of the vibration member and, thereby, to curb the above-stated drawbacks, a material which transmits sound at a low speed, for example, brass, is conventionally used for a vibration member including a contact spring. However, such a material degrades the frequency responsiveness of the contact spring, and thus fails to achieve smooth contact with the rotor. FIG. 9 shows the displacement pattern of a contact point P2 on the contact spring, which is experimentally determined. The displacement pattern is significantly different from the desirable pattern shown in FIG. 8. The pattern shown in FIG. 9 indicates that the rotor jumps and produces sounds during operation.
To overcome this drawback, a conventional contact spring is formed of a material which transmits sound at a high speed. However, if a contact spring 1b formed of a fast sound transmission material, such as aluminium, is connected to a vibration member 1 as shown in FIG. 10, then the vibration is considerably damped at the connecting portion. Therefore, this construction suffers deterioration in motor efficiency as well as an increase in production cost. In another conventional construction as shown in FIG. 7, aluminium or the like is used to form a component 1c of the vibration member 1. However, in this construction, because the driving frequency of the vibration member is increased, the required frequency responsiveness of the contact spring becomes severe and therefore difficult to achieve. Therefore, this construction fails to achieve substantial improvements or advantages.